From my personal experience - I've seen people that do good trades, then start to scam, because they get scammed their selfs and then they want to revenge on others, so they start scamming. It's quite sad, but has a point in it. It's like a never ending circle that I think can't be stopped. Another serious problem is that people don't take pics when they are trading, then they just put a text log to it's thread and the scammer ignores him, calls a "motherfucker" and then another dude gets scammed, until one dude makes some pictures of proof (like 3rd-5th dude that's getting scammed by the same person). That's how it works in Membership Sales. Also, Middle Manning is not really a great choice in trades as they take lots of time and usually take a fee that is worth as the whole trade. I've seen people that scam and at the same moment don't scam.. Pretty strange, but that happens and it happens quite a lot.

Basicly, the whole Black Market system got ruined by Jagex so called great update, when trading system went off. If it would be still on, then people could still trade with Middle Mans as the trades wouldn't take that long. (I still have a hope that they will return Trading system back)
To not scam - you have to have a high sense of HONOR, you have to want to be respected by others.
To sum up, it's a never ending circle that, in my opinion, can't be stopped.